(1) Field of the Invention
The present invention relates to flow noise reduction in towed line arrays and more particularly to an optimum flow noise cancelling hydrophone module which rejects turbulence induced flow noise while allowing acoustic signal to pass undisturbed at all relevant frequencies.
(2) Description of the Prior Art
The Navy is interested in detection and classification of low frequency, narrowband, noise emitting sources using high speed towed arrays. However, at high tow speeds the predominant low frequency noise is often flow related with levels which increase proportionally with speed. Typically the turbulence induced pressure fluctuations associated with the flow have wavenumbers, (2.pi./.lambda.), that are much larger than acoustical wavenumbers. These large wavenumbers correspond to small wavelengths. Moreover, the distance over which the flow noise is correlated is on the order of inches. Therefore, in the usual array of acoustically spaced hydrophones, high wavenumber noise voltage is independent between hydrophones. The flow noise level and the sonar figure of merit degradation are therefore proportional to tow vessel speed. Summing of grouped hydrophones (clustering) spaced within the noise coherence distance has been the main method of rejecting flow type noise up until recently. A big advantage of clustering over alternatives is the electronic simplicity--a resistor summing network. However, clustering does not reject flow noise adequately at frequencies where the noise wavelength is greater than the cluster length. In contrast, a flow noise cancelling hydrophone module is a bit more complex electronically but cancels correlated flow noise over a broader range of frequencies using signal processing techniques.
The instant invention emphasizes optimal signal processing methods which make use of the correlatedness of flow noise to cancel it from hydrophones used in mobile passive towed arrays. Gains in signal-to-noise ratio in my flow noise cancelling hydrophone module, U.S. patent application Ser. No. 103,827, filed Dec. 7, 1979, adequately rejected turbulent flow noise but also cancelled signals at some frequencies. My new apparatus rejects turbulent flow noise while passing the acoustic signal undisturbed, at all relevant frequencies. Thus the gain in signal-to-noise ratio equals the amount, in dB, of the noise reduction.